An ammonia combustion engine that uses ammonia as a fuel is known from the past. This ammonia combustion engine has a poor ignition property of ammonia, so that the combustion of ammonia at the time of low-load operation and at the time of high-load operation of the engine will be insufficient. Therefore, a combustion aid must be added in order to support the combustion of ammonia. As the combustion aid, a hydrocarbon fuel or hydrogen can be used. Here, ammonia is a compound made of hydrogen atoms and a nitrogen atom, so that hydrogen can be produced by chemical decomposition of ammonia. Therefore, when hydrogen produced by decomposition of ammonia is used as a combustion aid, the engine can be driven simply with ammonia alone, so that it can be considered as the most desirable system.
In order to produce hydrogen by decomposition of ammonia, for example, ammonia must be brought into contact with a ruthenium ammonia decomposition catalyst at a temperature of 400° C. or higher. This ammonia decomposition reaction is as follows.2NH3→N2+3H2  (1)
However, since this ammonia decomposition reaction is an endothermic reaction, heat must be supplied from the outside in order to obtain a stable ammonia decomposition ratio. In the case of an ammonia combustion engine automobile, engine exhaust gas is used as this heat source.
The following patent document 1 discloses an ammonia combustion engine in which the exhaust gas after combustion of ammonia in the ammonia combustion engine is supplied to ammonia decomposition reaction means. In this ammonia combustion engine, the ammonia decomposition reaction is promoted by utilizing the fact that the exhaust gas from the ammonia combustion engine has a high temperature.